Seasonally, the data evaluated here basically represent autumn (September and October) conditions, only a single data set derived from April. During summer, abundance of recruits may strongly deviate from the patterns described here because larvae often settled in areas where they could not survive until autumn, with summer storm events as a probable cause for the decline in many species . Thus, summer sampling including recent settlers may result in different patterns.
Grab type and size may also affect the distributional patterns detected: due to different sieving characteristics of large and small sediment volumes, the average abundances of species derived from the small-sized box cores used here may deviate from data obtained from larger grabs. Generally, larger cores are expected to better represent large-sized fauna while small cores may better represent the small-sized fauna because they require less vigorous sieve movements, with less risks of smashing small individuals, and sieving is faster, which means less time for the smaller individuals to escape through the meshes. This particularly applies to coarse grained sediments.
All data evaluated here come from the eastern North Sea off the North Frisian Islands from water depths between 10 and some 50 m. A majority of species correlated with water depth which may be mainly caused by local hydrography restricting many species towards the more turbulent shallows . Thus, the resulting distributional patterns over depth classes are shaped by hydrography and can only be generalised to hydrographically similar parts of the North Sea. The study area is characterised by an extended slope to 50 m depth; from local bathymetry  I therefore assume the data derived from here may also be transferable to the adjoining Danish coastline in the north, and down to the Netherlands towards the south. Whether or not they also fit for the SE of the British coast or even the entire eastern coast of Britain with a far steeper slope remains to be tested.
Hydrography of the adjoining Wadden Sea is quite different from the North Sea because the chain of shelter Islands strongly reduces wave heights while tidal currents are reduced in the shallow parts of the Wadden Sea but may be enhanced in the tidal inlets. Together with other factors such as a higher variability of temperature and salinity, this causes major differences in the levels of water depth and sediment type exploited by a species. Hence, the range of factors that actually limit the distributional ranges of a species may be different which, in the end, is the reason for differences in faunal composition between the Wadden Sea and the adjoining North Sea. This is also seen in the Lanice conchilega example above. While abundance of this species sharply dropped close to zero in the shallows of the wave-exposed study area, this species may be highly abundant in the shallows of the Wadden Sea, with winter frost as a limiting factor for its onshore distribution . Likewise, the results from the eastern North Sea cannot be generalised for the Baltic Sea because hydrography (in particular, tidal range) is different and because salinity is strongly decreased. The latter factor is known to cause dislocations towards deeper water in many species (brackish water submergence, [9, 10].
With respect to sediment composition, most species show clear preferences but are not confined to a single class of grain size, i.e. there seem to be no sediment-type specialists among the macrobenthic species of the study area. However, there are two restrictions to that statement. One the one hand, we do not know how the areas surrounding the species-specific high-abundance spots contribute to population recruitment; in some cases, abundance may be too low for a self-sustaining sub-population, so these areas may depend on recruits deriving from the high-abundance areas. On the other hand, this evaluation only included species that had been recorded 10 times, at least, in the current data set. However, the more a species is restricted to a single habitat type, the lower is the chance to record this species. Thus, sediment-type specialists may have passed this evaluation undetected.
Since the non-existence of an effect is impossible to proof, we do not know whether there are sediment-type generalists in a strict sense. But the data show that some species tolerate rather wide ranges of sediment types, mostly coinciding with a good swimming ability. Accordingly, crustaceans are particularly frequent among these species.
While the water depth occupied by a species in the study area is closely linked with local hydrography, the sediment types occupied may be easier generalised beyond the local level. This is expected because species-specific properties such as mode of movement, tube-building, food type and food acquisition may already restrict each species to a specific spectrum of sediment types. As an example, the polychaete Lanice conchilega mentioned above builds a tube from fine sand grains; accordingly, problems may arise both in pure mud and coarse sand if there are not enough sand grains of a fitting size available. However, median grain size is a rough classification of sediment type; other aspects of sediment composition such as mud content or sorting may be equally important for some species. In Lanice conchilega, for example, a fine sand content of a few percent may be enough to enable tube building in both mud and very coarse sand. Accordingly, poorly sorted coarse sand may be suitable while very well sorted coarse sand may not, despite of equal median diameters of the sand grains.
Generally, sediment sorting is thought to reflect sediment mobility  and hence hydrodynamic forcing. Thus, hydrodynamics affects the benthic fauna both directly and indirectly by shaping the properties of the sedimentary environment, though probably at different temporal scales. For direct effects, I propose short-term extremes like storm driven sediment disturbance to exert the strongest effect while the longer-term dynamics determines sediment composition. Thus, for some species the apparent sediment ‘preferences’ may be merely a proxy for the longer-term hydrodynamics of their environment and the physical factors associated with hydrodynamics.